Nitrogenase (N2ase) is a metalloenzyme that mediates biological nitrogen fixation and as such is essential to sustained life. Understanding the chemical mechanism by which N2ase-mediated nitrogen reduction occurs is therefore of general interest. The proposed program is to study biomimetic LnFe-Nx model complexes that will help to evaluate a mechanistic scheme that stresses a single Fe-Nx binding site in the FeMo cofactor (FeMoco) of N2ase. The experimental design focuses on functional rather than structural models of the FeMoco. Low molecular weight LnFe-Nx complexes will be developed to isolate a single iron site in either a tetrahedral or a trigonal bipyramidal geometry while at the same time preserving one binding site to accommodate dinitrogen and various other Nx functionalities. The Ln donor scaffolds will incorporate phosphorous and/or sulfur donor groups to render the LnFe species sufficiently electron-rich to bind relatively inert N2. By analogy to the mode of biocatalytic O2 reduction (FeII-O2 + 2 e-+ 2 H+ -> FeIV= O + H2O), a mechanistic sequence for N2 reduction that proceeds through a terminal nitride intermediate (Fel-N2 + 3 e-+ 3 H+ -> FeIV=(N + NH3) will be explored. To this end each of the critical intermediates envisaged for the complete cycle will be generated and studied. These intermediates will include Fel-N2, Fell-N=NH, Felll=N-NH2, FeIV = N, FeIII(NH, FeII-NH2, and Fel-NH3 species. Of further interest will be to understand how the local geometry and electronic structure of the LnFe-Nx species control their relative stabilities and reactivity patterns at their Fe-Nx linkages. The intent is to then use this knowledge to further design model systems that can facilitate catalytic N2ase activity. To accomplish these collective goals will require the physical and theoretical characterization of each type of Fe-Nx species, a mechanistic understanding of each of the stepwise transformations that pertain to the proposed cycle, and exploratory catalytic studies that will expose promising model systems that successfully mediate nitrogen fixation.